The exact molecular structures of some highly toxic polyhalogenated aromatic hydrocarbons have been obtained by X-ray crystallographic measurements or estimated based on postulated structures. The requirements of molecular symmetry and size as determined by the number, kind, and positions of halogens, planarity, interatomic distances and overall stereoelectronics suggest that a specific biological receptor may be involved which could account for the common toxic pattern. An underlying factor in the apparent symmetry requirement for biological activity and toxicity in these compounds appears to be net molecular polarizability. The guinea pig will continue to be used as a screening animal model in providing direction for further biological study of related compounds. A molecular level binding event which correlates with in vivo potency is being sought.